Eric,
welcome to APUG.

It is sometimes useful to know some "basics" in order to understand why certain things are the way they are.

VC-Paper is built of two emulsion layers with different spectral sensitivity and different gamma (grade). As long as you stay with grades 1-3, both layers are exposed in a way that the overall speed stays the same. At grades 00/0 and grades 4/5, basically only one layer, with each about half the speed, is exposed.